A polycrystalline silicon has been widely used for one type of semiconductor substrates for forming a solar battery and the production volume thereof has been rapidly increased every year. Such a polycrystalline silicon is generally formed by pouring and solidifying a silicon melt heated to melt at high temperature into a mold of a quartz crucible, a divisible graphite crucible or a quartz crucible accommodated into a graphite crucible, of which the inner surface of which is applied with a mold release material using a spray, a brush or a spatula or by adding a silicon raw material to a mold followed by melting once and then solidifying again.
The mold release material prevents a silicon ingot from contaminating with impurities or prevents a silicon melt from adhering to the inner wall surface of a crucible for casting, and plays an important role to release a solidified silicon ingot from a mold. Such a mold release material used includes a high-purity powder of silicon nitride, silicon carbide, and silicon oxide or a mixed power thereof in terms of generally a high melting point of the powder thereof and low contamination of a silicon ingot. In the past, many researches have been carried out to develop a method of forming a mold release material layer on the inner surface of a mold, a method of producing a mold with such treatment, and a method of producing a silicon ingot using the mold thereof in order to increase the productivity of a silicon ingot.
For example, Patent Literature 1 describes that in production of a silicon ingot, peeling of parts of a mold release layer making a silicon melt contact with a mold and contamination of a silicon melt with a peeled product are effectively prevented by eliminating in advance aggregation of silicon nitride particles in a slurry using a silicon nitride powder SN-E10 made of Ube Industries, Ltd. produced by an imide thermal decomposition method, thereby uniformly adhering a slurry including silicon nitride particles coated with the oxide film to the inner surface of a mold base body so as to make the surface of the mold release layer flat, and using the mold with the mold release layer formed herewith for production of a silicon ingot.
Patent Literature 2 also describes, for example, a method of forming a mold release layer having a two-layer structure in which the density is adjusted in order to provide both good adhesion to a mold and good release properties of a silicon ingot, and a method of fusing the mold release material containing silicon oxide added thereto.